Sunday's tornado ripped into a hospital, a nursing home and thousands of other buildings, killing at least 116 in what's being called the worst twister ever to hit Missouri.

Amy Langford carries belongings from her house in Joplin, Mo. She and her… (Larry W. Smith, European…)

Reporting from Joplin, Mo. — When the tornado hit, Staci Perry, a scrub technician at St. John's Regional Medical Center, had just left the operating room to grab a piece of equipment for a surgery in progress. An urgent announcement came over the loudspeaker: "Execute condition gray." That was the hospital's code for an impending disaster, though in drills, the command was always preceded by "Prepare for condition gray."

There was no time to prepare. As she heard the massive glass walls crack, Perry, 33, dashed back to surgery. "The pressure in everyone's ears was just tremendous," she said. A physician's assistant threw himself against the door so it wouldn't blow in and destroy the operating room. The lights went out. The wind howled.

"Literally, the hospital imploded," said Dr. Jim Riscoe, an emergency room physician at the 230-bed facility. There is an emergency plan for disasters, he said, "but they don't anticipate the emergency being the hospital."

When it was over, just after 5:30 p.m. Sunday, the storm had gouged a six-mile swath roughly half a mile wide in this city of 50,000 people. At least 116 people died, five of them hospital patients.

The apocalyptic after-images were depressingly familiar, reminiscent of those from the deadly April tornadoes in the South: rubble as far as the eye could see, cars buried under pieces of houses, trees wrenched from the ground with massive roots reaching toward the sky, columns of smoke rising from gas fires, emergency vehicles with lights flashing. And everywhere, knots of people stunned by nature's violence mourned their losses, counted their blessings and told their harrowing stories.

Joplin officials said more than 2,000 structures were ripped apart and whole neighborhoods obliterated in what was described as the worst tornado ever to hit Missouri. Power remained out Monday on most of the city's west side. Residents were advised to boil water.

"We still believe there are people to be saved in the rubble," said Missouri Gov. Jay Nixon, who deployed about 140 National Guard troops to help with rescue efforts.

President Obama, visiting Ireland, expressed his condolences in a telephone call to Nixon. Federal Emergency Management Agency Administrator Craig Fugate was on his way to Joplin to coordinate federal disaster relief, said White House spokesman Nick Shapiro.

The National Weather Service said Monday that the Joplin tornado was the deadliest single tornado since a 1953 twister killed 116 in Flint, Mich. With winds as high as 198 mph, it was rated F-4 on the Fujita scale, one step below the strongest tornado. (Multiple tornadoes were responsible for approximately 344 deaths over four days in late April, mostly in Alabama.)

The weather service also said that more than 100 tornadoes had occurred during May, the most active month for tornadoes. The May record of 542 tornadoes was set in 2003.

Another twister touched down Sunday in north Minneapolis, Minn., killing one person.

Dr. Jason Persoff, 39, a Florida internist and storm chaser, had been in southeast Kansas on Sunday with his storm-chasing partner, also a physician, when they realized they were seeing something huge. "This storm will do incredible things," Persoff remembered thinking.

But as the pair headed into Missouri, they heard on the radio that a "debris ball" had been spotted on radar — a mass of material torn from the ground, carried along by the turbulence. Persoff's excitement turned to alarm. He pulled off the highway into Joplin and saw crumpled semi trucks. Flagging down an emergency worker, he got directions to the nearest functioning hospital, where he and his friend helped treat patients all night, amazed at the dedication of the hospital staff.

"They didn't know what happened to their families," Persoff said, "and yet they were focused 190% on keeping people alive."

A few blocks from St. John's hospital, where a helicopter had been blown off the roof, Zach Simonds was cleaning floors at the Greenbrier Nursing Home when he heard a "code red" announcement on the intercom. About a dozen staffers tried to gather up the 85 patients into the central hall, since the home had no basement. Some refused to budge. But most clustered in the hall.

The tornado tore the roof off the nursing home. Simonds could see cars being tossed overhead. "Everybody was praying; you could hear people praying, 'Please God don't kill me,' " he said. Then the glass-plated front of the building burst.